1. Field of the Invention
The present invention relates to a method of and an apparatus for manufacturing optical fibers, and more particularly to a method of and an apparatus for manufacturing erbium-doped optical fibers usable as an optical amplifier allowing optical signals to be directly amplified by themselves, which method and apparatus are capable of reducing manufacturing time while increasing productivity.
2. Description of the Related Art
Where a long-distance signal transmission is performed in an ultrahigh-speed information communications network or in a long-distance communications network or where an optical signal generated at one area branches off in various directions in such a communications network, the optical signal is reduced in intensity during its transmission from its initial intensity. Accordingly, it is necessary to greatly amplify the optical signal. In order to satisfy such a necessity, semiconductor amplifiers or optical amplifiers are employed. In particular, semiconductor amplifiers have been widely used as an essential element of ultrahigh-speed information communications networks because they can directly amplify optical signals to a desired high level without requiring a complicated signal processing.
Such optical amplifiers employ optical fibers such as optical fibers containing erbium (Er) which is a medium serving to internally amplify an optical signal. Such erbium-doped optical fibers may be manufactured using various methods. One such method is described in U.S. Pat. No. 5,526,459 to Daiichirou Tanaka, et al. entitled Erbium-Doped Silica Optical Fiber Preform which describes making an Erbium-doped optical fiber using the VAD (Vapor-phase axial-deposition) process to dope the optical fiber. The most frequently used and reliable method, however, is a modified chemical vapor deposition (MCVD) method as described in U.S. Pat. No. 5,284,500 to Kouji Okamura, et al., entitled Process For Fabricating An Optical Fiber Preform.
The following description will be made in conjunction with the case wherein erbium-doped optical fibers are manufactured using a MCVD method. In accordance with the method using the MCVD method, a connecting tube is first clamped at one end thereof on a clamping chuck, a quartz tube, which is called "a supporting tube", is connected at the other end of the connecting tube, wherein the quartz tube is used to manufacture an erbium-doped optical fiber substrate. Thereafter, raw material such as SiCl.sub.4 or GeCl.sub.4 transported from a raw supply system by a flow of oxygen is supplied to the interior of the quartz tube. Subsequently, the quartz tube is heated by an external heating source (for example, an oxygen/hydrogen burner) while rotating. During the heating process, an oxidation reaction occurs in the interior of the quartz tube.
In accordance with the oxidation reaction, particles of quartz-based oxides containing impurities are produced. The oxide particles exist in the form of a deposition on the inner surface of the quartz tube. As the heating process is further carried out while the heating source reciprocates in a longitudinal direction on the quartz tube, the particulate deposition is sintered on the inner surface of the quartz tube while being transparentized. As a result, a thin glass layer is formed on the inner surface of the quartz tube. Thereafter, the above procedure is repeated until the glass layer on the inner surface of the quartz tube has a desired thickness. During the formation of the glass layer, a portion of the glass layer corresponding to a clad layer is first formed, and a portion of the glass layer corresponding to a core layer is then formed.
In order to manufacture erbium-doped optical fibers capable of directly amplifying optical signals to a desired high level without requiring a complicated signal processing by use of the quartz tube formed with the clad layer and core layer, the quartz tube is separated from the clamping chuck after being closed at its one end. Thereafter, a solution containing erbium and other additive elements is injected into the interior of the quartz tube closed at one end thereof. The quartz tube is then maintained in the above-mentioned condition for a desired period of time so as to allow the erbium to be absorbed in the core layer to a desired amount. After a desired period of time elapses, the solution is removed from the quartz tube. At this time, the core layer has absorbed the solution containing the erbium and other additive elements. Subsequently, the quartz tube is clamped again on the clamping chuck, and its closed end is then opened. The clamping chuck then rotates to rotate the quartz tube so as to prevent the solution absorbed in the core layer from being sporadically concentrated in the core layer. Thereafter, the quartz tube is maintained for a long period of time as it is, so that the solution absorbed in the quartz tube can be air-dried. After the erbium absorbed in the quartz tube is completely dried in the above process, the quartz tube is heated again at a high temperature using the heating source, so that it is softened. Thereafter, both ends of the quartz tube are completely sealed. Thus, an erbium-doped optical fiber substrate having a hollow cylindrical structure is obtained. However, the above-mentioned method involving the step of drying the solution containing the erbium and other additive elements absorbed in the quartz tube is problematic is in that a long period of time is required to carry out the drying step because the drying step is processed in a natural air-dried state. This results in a lengthened manufacturing time of erbium-doped optical fiber substrates. As a result, the manufacturing productivity of erbium-doped optical fiber substrates is degraded. Furthermore, the costs of erbium-doped optical fiber substrates increase. In addition, a sporadic undrying phenomenon may occur because the quartz tube is dried in a natural air-dried state. Such a sporadic undrying phenomenon results in a non-uniform refractivity distribution.
Another method for manufacturing a substrate of erbium-doped optical fibers using the MCVD method will now be described. In accordance with this method using the MCVD method, a connecting tube is first clamped at one end thereof on a clamping chuck. A quartz tube, which is called "a supporting tube", is connected at the other end of the connecting tube. The quartz tube is used to manufacture an erbium-doped optical fiber substrate. Thereafter, raw material such as SiCl.sub.4 or GeCl.sub.4 transported from a raw supply system by a flow of oxygen is supplied to the interior of the quartz tube. Subsequently, the quartz tube is heated by an external heating source (for example, an oxygen/hydrogen burner) while rotating. During the heating process, an oxidation reaction occurs in the interior of the quartz tube.
In accordance with the oxidation reaction, particles of quartz-based oxides containing impurities are produced. The oxide particles exist in the form of a deposition on the inner surface of the quartz tube. As the heating process is further carried out while the heating source reciprocates in a longitudinal direction on the quartz tube, the particle deposition is sintered on the inner surface of the quartz tube while being transparentized. As a result, a thin glass layer is formed on the inner surface of the quartz tube. Thereafter, the above procedure is repeated until the glass layer on the inner surface of the quartz tube has a desired thickness.
During the formation of the glass layer, a portion of the glass layer corresponding to a clad layer is first formed, and a portion of the glass layer corresponding to a core layer is then formed. In order to manufacture erbium-doped optical fibers capable of directly amplifying optical signals to a desired high level without requiring a complicated signal processing by use of the quartz tube formed with the clad layer and core layer, the quartz tube is separated from the clamping chuck after being closed at its one end. Thereafter, a solution containing erbium and other additive elements is injected into the interior of the quartz tube closed at one end thereof. The quartz tube is then maintained in the above-mentioned condition for a desired period of time so as to allow the erbium to be absorbed in the core layer to a desired amount. After a desired period of time elapses, the solution is removed from the quartz tube. At this time, the core layer has absorbed the solution containing the erbium and other additive elements. Subsequently, the quartz tube is clamped again on the clamping chuck, and its closed end is then opened. The clamping chuck then rotates to rotate the quartz tube so as to prevent the solution absorbed in the core layer from being sporadically concentrated in the core layer. During the rotation of the quartz tube, the outer surface of the quartz tube is slowly heated by a heater at a low temperature while the heater reciprocates in a longitudinal direction on the quartz tube, thereby causing the solution absorbed in the quartz tube to be slowly dried.
After the erbium absorbed in the quartz tube is completely dried in accordance with the above process, the quartz tube is heated again by a heating source at a high temperature, so that it is softened. Thereafter, both ends of the quartz tube are completely sealed. Thus, an erbium-doped optical fiber substrate having a hollow cylindrical structure is obtained. Where the solution containing the erbium and other additive elements absorbed in the quartz tube is dried in accordance with the above-mentioned method, it is possible to greatly reduce the drying time, as compared to the method using the natural air-drying process. This is because the outer surface of the quartz tube is slowly heated by the heater at a warm temperature while the heater reciprocates in a longitudinal direction on the quartz tube, thereby causing the solution absorbed in the quartz tube to be dried. Even in this case, however, several hours are required to dry the quartz tube. As a result, this method is also problematic in that a long period of time is taken to manufacture erbium-doped optical fiber substrates. As a result, the manufacturing productivity of erbium-doped optical fiber substrates is degraded. Furthermore, the costs of erbium-doped optical fiber substrates increase. In addition, a sporadic undrying phenomenon may occur because the quartz tube is dried using heat generated by the heater. Such a sporadic undrying phenomenon results in a non-uniform refractivity distribution. Since the heater is used to dry the quartz tube, additional time and costs are required to install the heater. Moreover, an erroneous installation of the heater may cause errors in the manufacture of erbium-doped optical fiber substrates.
U.S. Pat. No. 5,296,012 to A. Joseph Antos, et al., entitled Method Of Making Optical Waveguide Preforms and U.S. Pat. No. 5,314,518 to Masumi Ito, et al., entitled Method For Producing Glass Preform For Optical Fibers utilize known MCVD methods for doping optical fibers with a rare earth material such as Erbium.